Unitary element: Difference between revisions
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In [[mathematics]], an element |
In [[mathematics]], an [[Element (mathematics)|element]] of a [[*-algebra]] is called '''unitary''' if it is [[Inverse element|invertible]] and its inverse element is the same as its adjoint {{nowrap|element.{{sfn|Dixmier|1977|p=5}}}} |
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== Definition == |
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In [[functional analysis]], a [[linear operator]] ''A'' from a [[Hilbert space]] into itself is called '''unitary''' if it is invertible and its inverse is equal to its own [[adjoint operator|adjoint]] ''A''{{sup|∗}} and that the domain of ''A'' is the same as that of ''A''{{sup|∗}}. See [[unitary operator]] for a detailed discussion. If the Hilbert space is finite-dimensional and an [[orthonormal basis]] has been chosen, then the operator ''A'' is unitary if and only if the [[matrix (mathematics)|matrix]] describing ''A'' with respect to this basis is a [[unitary matrix]]. |
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Let <math>\mathcal{A}</math> be a *-Algebra with [[Identity element|unit]] {{nowrap|<math>e</math>.}} An element <math>a \in \mathcal{A}</math> is called unitary if {{nowrap|<math>aa^* = a^*a = e</math>.}} In other words if <math>a</math> is invertible and <math>a^{-1} = a^*</math> {{nowrap|holds.{{sfn|Dixmier|1977|p=5}}}} |
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The [[Set (mathematics)|set]] of unitary elements is denoted by <math>\mathcal{A}_U</math> or {{nowrap|<math>U(\mathcal{A})</math>.}} |
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A special case from particular importance is the case where <math>\mathcal{A}</math> is a [[Banach algebra#Banach *-algebras|complete normed *-algebra]], that satisfies the C*-identity (<math>\left\| a^*a \right\| = \left\| a \right\|^2 \ \forall a \in \mathcal{A}</math>), which is called a [[C*-algebra]]. |
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== Criteria == |
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* Let <math>\mathcal{A}</math> be a [[Algebra over a field#Unital algebra|unital]] C*-algebra and <math>a \in \mathcal{A}_N</math> a [[Normal element|normal]] element. Exactly then is <math>a</math> unitary if the [[Banach algebra#Spectral theory|spectrum]] <math>\sigma(a)</math> consists only of elements of the [[circle group]] <math>\mathbb{T}</math>, i.e. {{nowrap|<math>\sigma(a) \subseteq \mathbb{T} = \{ \lambda \in \C \mid | \lambda | = 1 \}</math>.{{sfn|Kadison|1983|p=271}}}} |
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== Examples == |
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* The unit <math>e</math> is {{nowrap|unitary.{{sfn|Dixmier|1977|pages=4-5}}}} |
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Let <math>\mathcal{A}</math> be a unital C*-algebra, then: |
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* Every [[Projection (mathematics)|projection]], i.e. every element <math>a \in \mathcal{A}</math> with <math>a = a^* = a^2</math>, is unitary. For the spectrum of a projection consists of at most <math>0</math> and <math>1</math>, as follows from the {{nowrap|[[continuous functional calculus]].{{sfn|Blackadar|2006|pages=57,63}}}} |
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* If <math>a \in \mathcal{A}_{N}</math> is a normal element of a C*-algebra <math>\mathcal{A}</math>, then for every [[continuous function]] <math>f</math> on the spectrum <math>\sigma(a)</math> the continuous functional calculus defines an unitary element <math>f(a)</math>, if {{nowrap|<math>f(\sigma(a)) \subseteq \mathbb{T}</math>.{{sfn|Kadison|1983|p=271}}}} |
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== Properties == |
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Let <math>\mathcal{A}</math> be a unital *-algebra and {{nowrap|<math>a,b \in \mathcal{A}_U</math>.}} Then: |
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* The element <math>ab</math> is unitary, since {{nowrap|<math display="inline">((ab)^*)^{-1} = (b^*a^*)^{-1} = (a^*)^{-1} (b^*)^{-1} = ab</math>.}} In particular, <math>\mathcal{A}_U</math> forms a {{nowrap|[[multiplicative group]].{{sfn|Dixmier|1977|p=5}}}} |
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* The element <math>a</math> is {{nowrap|normal.{{sfn|Dixmier|1977|pages=4-5}}}} |
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* The adjoint element <math>a^*</math> is also unitary, since <math>a = (a^*)^*</math> holds for the [[Involution (mathematics)|involution]] {{nowrap|*.{{sfn|Dixmier|1977|p=5}}}} |
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* If <math>\mathcal{A}</math> is a C*-algebra, <math>a</math> has norm 1, i.e. {{nowrap|<math>\left\| a \right \| = 1</math>.{{sfn|Dixmier|1977|p=9}}}} |
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== See also == |
== See also == |
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⚫ | |||
* {{annotated link|Normal element}} |
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* [[Unitary operator]] |
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* {{annotated link|Self-adjoint}} |
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⚫ | |||
== |
== Notes == |
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{{reflist}} |
{{reflist}} |
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== References == |
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*{{cite book |author-link=Michael C. Reed |first=M. |last=Reed |author-link2=Barry Simon |first2=B. |last2=Simon |title=Methods of Mathematical Physics |others=Vol 2 |publisher=Academic Press |year=1972 }} |
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*{{cite book | |
* {{cite book |last=Blackadar|first=Bruce |title=Operator Algebras. Theory of C*-Algebras and von Neumann Algebras |publisher=Springer |location=Berlin/Heidelberg |year=2006 |isbn=3-540-28486-9 |pages=57,63 }} |
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* {{cite book |last=Dixmier |first=Jacques |title=C*-algebras |publisher=North-Holland |location=Amsterdam/New York/Oxford |year=1977 |isbn=0-7204-0762-1 |translator-last=Jellett |translator-first=Francis }} English translation of {{cite book |display-authors=0 |last=Dixmier |first=Jacques |title=Les C*-algèbres et leurs représentations |language=fr |publisher=Gauthier-Villars |year=1969 }} |
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* {{Schaefer Wolff Topological Vector Spaces|edition=2}} <!-- {{sfn | Schaefer | Wolff | 1999 | p=}} --> |
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* {{cite book |last1=Kadison |first1=Richard V. |last2=Ringrose |first2=John R. |title=Fundamentals of the Theory of Operator Algebras. Volume 1 Elementary Theory. |publisher=Academic Press |location=New York/London |year=1983 |isbn=0-12-393301-3}} |
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{{SpectralTheory}} |
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{{Functional Analysis}} |
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[[Category:Abstract algebra]] |
[[Category:Abstract algebra]] |
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[[Category: |
[[Category:C*-algebras]] |
Revision as of 15:12, 12 January 2024
In mathematics, an element of a *-algebra is called unitary if it is invertible and its inverse element is the same as its adjoint element.[1]
Definition
Let be a *-Algebra with unit . An element is called unitary if . In other words if is invertible and holds.[1]
The set of unitary elements is denoted by or .
A special case from particular importance is the case where is a complete normed *-algebra, that satisfies the C*-identity (), which is called a C*-algebra.
Criteria
- Let be a unital C*-algebra and a normal element. Exactly then is unitary if the spectrum consists only of elements of the circle group , i.e. .[2]
Examples
- The unit is unitary.[3]
Let be a unital C*-algebra, then:
- Every projection, i.e. every element with , is unitary. For the spectrum of a projection consists of at most and , as follows from the continuous functional calculus.[4]
- If is a normal element of a C*-algebra , then for every continuous function on the spectrum the continuous functional calculus defines an unitary element , if .[2]
Properties
Let be a unital *-algebra and . Then:
- The element is unitary, since . In particular, forms a multiplicative group.[1]
- The element is normal.[3]
- The adjoint element is also unitary, since holds for the involution *.[1]
- If is a C*-algebra, has norm 1, i.e. .[5]
See also
Notes
- ^ a b c d Dixmier 1977, p. 5.
- ^ a b Kadison 1983, p. 271.
- ^ a b Dixmier 1977, pp. 4–5.
- ^ Blackadar 2006, pp. 57, 63.
- ^ Dixmier 1977, p. 9.
References
- Blackadar, Bruce (2006). Operator Algebras. Theory of C*-Algebras and von Neumann Algebras. Berlin/Heidelberg: Springer. pp. 57, 63. ISBN 3-540-28486-9.
- Dixmier, Jacques (1977). C*-algebras. Translated by Jellett, Francis. Amsterdam/New York/Oxford: North-Holland. ISBN 0-7204-0762-1. English translation of Les C*-algèbres et leurs représentations (in French). Gauthier-Villars. 1969.
- Kadison, Richard V.; Ringrose, John R. (1983). Fundamentals of the Theory of Operator Algebras. Volume 1 Elementary Theory. New York/London: Academic Press. ISBN 0-12-393301-3.